Using Roles

In the previous lessons, you've used OpenZeppelin's Ownable contract to implement a simple access control mechanism based on the concept of ownership. Simply put, only the owner was allowed to call the setLatestEthPrice function.

Now, to make the oracle more decentralized, we need to implement a system that provides different levels of access: owner and oracle. The owner should be able to add and remove oracles. In turn, an oracle must be allowed to update the ETH price by calling the setLatestEthPrice function.

Using Roles

Fortunately, OpenZeppelin provides a library called Roles that does all the heavy lifting. To you use it, you'll first have to import it using something like the following:

import "@openzeppelin/contracts/access/AccessControl.sol";

Now, that you imported from the Roles smart contract, let's see what makes it tick:

pragma solidity ^0.5.0;

/**
 * @title Roles
 * @dev Library for managing addresses assigned to a Role.
 */library Roles {
    struct Role {
        mapping (address => bool) bearer;
    }

/**
     * @dev Give an account access to this role.
     */function add(Role storage role, address account) internal {
        require(!has(role, account), "Roles: account already has role");
        role.bearer[account] = true;
    }

/**
     * @dev Remove an account's access to this role.
     */function remove(Role storage role, address account) internal {
        require(has(role, account), "Roles: account does not have role");
        role.bearer[account] = false;
    }

/**
     * @dev Check if an account has this role.
     * @return bool
     */function has(Role storage role, address account) internal view returns (bool) {
        require(account != address(0), "Roles: account is the zero address");
        return role.bearer[account];
    }
}

Note that Roles is a library. For us, this means that we can attach it to the Roles.Role data type like so:

using Roles for Roles.Role;

Once we do this, the first parameter expected by the add, remove, and has functions (that is Roles storage role) is automatically passed, meaning we can use these functions to manage our roles as follows:

oracles.add(_oracle);// Adds `_oracle` to the list of oracles
oracles.remove(_oracle);// Removes `_oracle` from the list of oracles
oracles.has(msg.sender);// Returns `true` if `msg.sender` is an `oracle`

Put It to the Test

1. In the tab to the right, replace the line of code that imports from the Ownable.sol file with the line of code that imports from the Roles.sol file.
2. Make it so that the contract doesn't inherit from Ownable by removing is Ownable from the line of code that declares the contract.
3. Attach Roles to the Roles.Role data type.
4. Declare a Roles.Role variable called owners. Make it private.
5. Similarly, create a Roles.Role variable called oracles. Don't forget to make it private as well.